Various types of energy storing systems are known. One type is based on flywheels, that is to say on at least one mass set in rotation by input of energy, which will continue to rotate, under inertia, after the energy input has ceased. The rotating mass is connected to a motor which constitutes a means of inputting energy during the energy-storage periods, or a generator during energy-restoring periods. The heavier the flywheel and the more able it is to rotate quickly with the lowest possible friction, the greater the amount of energy that can be stored. Mounting of the flywheel bearings is therefore of key importance.
In some types of flywheel the bearings are partially relieved of the weight of the flywheel by applying an electromagnetic force.
Another type of flywheel is described in PCT application PCT/NL2009/000248. This flywheel based inertial energy accumulation device comprises a frame and at least one flywheel mounted so that it can rotate relative to the frame about an axis of rotation, as well as means for exposing at least one face of the flywheel to a gas pressure which, by comparison with the pressure applied to a substantially opposite face of the flywheel, generates an upward differential pressure force that at least partially compensates for the weight of the flywheel, such as by means of so-called gas flow restricting means (also referred to as a seal in the present application) surrounding the face of the flywheel that is exposed to the gas pressure. It is stated that in this device not only the flywheel bearings are relieved at least partially of the weight of the flywheel, thus increasing their life, but the cost per kWh is also greatly reduced. These gas flow restricting means make it possible to create a drop in pressure head in the leakage space. These means are typically formed between the flywheel and a surface integral with the frame. In an embodiment these flow slowing means comprise a labyrinth seal. In such a seal, the gas flow path comprises a succession of special surface features that generate drops in pressure head (“head drops”). For example, the cross section for the passage of the gas is alternately reduced and enlarged.
In practice, stability and safety of this device leave something to be desired. In general, the vulnerability of the seal is high. In case of emergency, for example failure, defect, accident and the like serious damage to the precisely, often tailor made, special features of the seal is most likely to occur because of (friction) contact between the rotor and the seal. The rotor itself may also become damaged.
It has also appeared that in use there is a risk of unbalanced rotation and/or uncontrolled vibration of the rotor, e.g. in the direction of the rotation axis, which may in certain cases involve a self-attenuating effect. Uncontrolled vibration makes the device less energy efficient and may result in damage of the rotor and other components of the device, such as the seal and housing. This is highly undesirable.